In general, a solar cell is formed using a semiconductor wafer typified by a silicon wafer. In order to enhance the conversion efficiency of a solar cell, it is necessary to reduce light reflected by a light receiving surface of the solar cell and light transmitted by the solar cell. In order to improve the conversion efficiency, for example, in a case where a crystalline solar cell is fabricated using a silicon wafer, since silicon wafers transmit low percentage of visible light which contributes to photoelectric conversion; consideration should be given to reduction of reflection loss of visible light at the silicon wafer surface serving as the light receiving surface and effective confinement of incident light into the solar cell.
Techniques for reducing reflection loss of incident light at a silicon wafer surface include a technique of forming an antireflection film on the surface and a technique of forming irregular features such as a so-called textured structure having pyramid-shaped micro recesses and micro protrusions. The method of forming a textured structure on the surface, of the latter technique, is a method suitable for single crystal silicon, which is typified by a method of etching a (100) single crystal silicon surface with an alkaline solution. This method makes use of etching using alkali which causes slower etching rate on the (111) face than on the (100) face and the (110) face. Further, as the latter technique, a technique of reducing reflection loss of incident light by making a silicon surface porous to form irregular features on the surface has been proposed in recent years.
PTL 1 (JP 06-169097A) discloses a method of forming many micropores on a surface of a single crystal silicon substrate by anodization in which electric current is flown in hydrofluoric acid using the single crystal silicon substrate as an anode and Pt as a cathode. PTL 2 (JP 2004-5063301) discloses a method of forming a porous layer having many regular pores in the direction perpendicular to the wafer surface by reactive ion etching (RIE).
PTL 3 (JP 2007-194485A) discloses a method of forming, on a silicon substrate surface on which a micron-size textured structure has been formed, more microscopic irregularities of the submicron order by coating the surface with metal particles by electroless plating, and then etching the substrate with a mixed aqueous solution of an oxidant and hydrofluoric acid. Specifically, the alkali textured p-type single crystal silicon substrate is dipped in an aqueous solution containing silver perchlorate and sodium hydroxide to form silver microparticles on its surface. After that, the substrate is dipped in a mixed solution of a hydrogen peroxide solution, hydrofluoric acid, and water, thereby forming irregularities of the submicron order.